Metallic titanium materials have so far been used in a wide range of fields as structural materials owing to their high specific tensile strength, high corrosion resistance and other excellent characteristics. In particular, in the field of semiconductors where the integration degree has been markedly increased in recent years, the consumption of high-purity titanium materials is rapidly increasing to meet the demands from the fine pattern processing viewpoint since high-purity titanium materials have low resistance characteristics and high strength characteristics or since titanium nitride has barrier characteristics, for instance.
In cases where a high-purity titanium material is applied as a semiconductor material such as an electrode material, it is generally used as targets for sputtering and, in that case, the material is required to have a purity of not lower than 99.98%, without taking gaseous impurities into consideration.
In producing targets for sputtering, a forging material is subjected to cylindrical cogging and then to upset forging or axisymmetric rolling so that good sputtering characteristics can be secured and the working yield can be improved. More specifically, a titanium ingot (e.g. 730 mm φ) prepared by melting at a high purity is used as the material and subjected to two or more repetitions of the step of cylindrical cogging to a predetermined outside diameter (e.g. 165 mm φ). Then, the cogging is cut to a predetermined length and subjected to upset forging by compressing the same in the longitudinal direction to give a disk-shaped titanium material having a predetermined thickness. Then, for uniformly extending the thus-worked disk-shaped titanium material in radial directions, axisymmetric rolling is carried out to give a thinner disk-like shape (e.g. 25 mm thick). Thereafter, the disk is subjected to machine cutting and finishing to give desired targets for sputtering.
The metallographic structure of a titanium material may include the α phase close-packed hexagonal structure and the β phase body-centered cubic structure depending on the temperature conditions, and the transformation from the α phase to the β phase occurs at the β transformation point which is in the high temperature region. Meanwhile, among such metallographic structures, the body-centered cubic structure shows better workability as compared with the close-packed hexagonal structure but allows marked growth of crystal grains under high temperature conditions. Therefore, in working such material to give products, such as targets for sputtering, for which the growth of crystal grains and/or recrystallization is to be suppressed, it becomes necessary to control the working temperature therefor. In particular, the growth of crystal grains of high-purity titanium becomes more remarkable under high temperature conditions and, therefore, it becomes necessary to work at temperatures not higher than the β transformation point. Therefore, in cogging a cylindrical titanium material in the above-mentioned target manufacturing process, warm forging at temperatures not higher than the β transformation point is a prerequisite for suppressing the growth of crystal grains while securing a certain degree of workability.
FIG. 1 is a diagram illustrating the prior art warm forging process for cylindrically cogging a raw material ingot to a cylindrical final form. As shown in the figure, the cogging process from the raw material ingot (e.g. 730 mm φ) to a finish outside diameter of 165 mm φ includes 4 steps. In the prior art warm forging, even in the case of cogging to a cylindrical final shape, forging is carried out to a prism-like shape using flat molds in the middle stage of forging and thereafter, for example until the third step in which cogging is performed to a shape of 175 mm square.
In a typical forging process, a raw material ingot is uniformly heated in a heating furnace and then subjected to free forging using flat molds. As for typical means, the material is pressed in the vertical direction, then rotated by 90° or 270° and again pressed in the vertical direction. This procedure is repeated to give an intermediate material having a shape of 175 mm square. Thereafter, the four vertices of the intermediate material are respectively pressed to give an octagonal shape, and the resulting vertices are further pressed to a 16-sided figure, then to a 32-sided figure and so on, in a manner more and more approaching to a circle to finally give a cylindrical cogging with a finish outside diameter of 165 mm φ.
In manufacturing targets for sputtering, as mentioned above, cogging is performed to give a final cylindrical titanium material and this material is then cut to a predetermined length and then subjected to upset forging in the longitudinal direction to give a disk-shaped titanium material having a predetermined thickness. However, when cogging is performed in the prior art manner of warm forging, as shown in FIG. 1, the titanium material after upset forging cannot have a shape close to a circle but becomes irregular in deformation in radial directions.
FIG. 2 is a representation of irregularities in sectional deformation in radial directions as resulting from upset forging, FIG. 2 (a) showing a case of square deformation and (b) showing a case of elliptical deformation. In this instance, the cross section after upset forging has a major diameter or major axis portion LA and a minor diameter or minor axis portion LB and the ratio between them (LA/LB) is much greater than 1.0 and, thus, the cross section becomes quite different from a circular shape and the upset forgeability becomes worse. This tendency becomes more remarkable as the upsetting ratio increases.
In manufacturing targets for sputtering, the titanium material after upset forging is finished to a disk-like shape by axisymmetric rolling. However, it is difficult to correct the titanium material cross-section once given a shape differing from a circular one. If, therefore, the titanium material having a square or elliptical cross section is subjected as it is to target processing, great process losses will occur until final products are obtained and, accordingly, the yield of products will decrease markedly.
In some cases, high-speed forging is employed in cylindrical cogging by warm forging. For working of titanium, in particular, a type GFM high-speed forging machine has been developed. On this forge, a material is pounded simultaneously from four directions by means of four reciprocating hammers and the material to be forged is fed thereto by means of chuck heads provided before and after the hammers while being rotated.
However, when such high-speed forging is employed, the working force supplied by the molds, such as hammers, tends to fail to extend to the central portion of the material, leading to preferential working of the material surface. Thus, the so-called fish tail phenomenon may become significant, and the worked structure of the cogging also tends to become inhomogeneous and thus the cross section will show increased unevenness in grain size in radial directions. For such reasons, high-speed forging cannot be employed as means of working of targets for sputtering.